One step ultrasonically coated substrate for use in a capacitor

ABSTRACT

A deposition process for coating a substrate with an ultrasonically generated aerosol spray of a pseudocapacitive material or a precursor thereof contacted to a substrate heated to a temperature to instantaneously solidify the pseudocapacitive material or convert the precursor to a solidified pseudocapacitive metal compound, is described. The ultrasonic aerosol droplets are much smaller in size than those produced by conventional processes and the heated substrate minimizes the possibility of contamination, thereby providing the present coating having an increased surface area. When the coated substrate is an electrode in a capacitor, a greater surface area results in an increased electrode capacitance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a deposition process forcoating a substrate with an ultrasonically generated aerosol spray. Moreparticularly, the present invention relates to a metallic foil providedwith an ultrasonically generated aerosol spray. Still more particularly,the present invention provides a porous, high surface area metal oxide,metal nitride, metal carbon nitride or metal carbide coating on aconductive foil for use in a capacitor and the like.

2. Prior Art

In redox active structures, energy storage occurs during a change in theoxidation state of the metal when an ionic species from a conductingelectrolyte, for example a proton, reacts with the surface or bulk ofthe oxide. This chemisorption is accompanied by the simultaneousincorporation of an electron into the oxide. The surface (or bulk)interaction between the electrode and electrolyte gives rise tocapacitance in the hundreds of μF/sq.cm. It follows that a electrodewith high specific surface area will store a significant amount ofenergy and will have a large specific capacitance. These electrodes arethen appropriate when used as the anode and/or cathode inelectrochemical capacitors or as cathodes in electrolytic capacitors,which require high specific capacitances.

Whether an anode or a cathode in an electrochemical capacitor or thecathode in an electrolytic capacitor, a capacitor electrode generallyincludes a substrate of a conductive metal such as titanium or tantalumprovided with a semiconductive or pseudocapacitive oxide coating,nitride coating, carbon nitride coating, or carbide coating. In the caseof a ruthenium oxide cathode, the coating is formed on the substrate bydissolving a ruthenium oxide precursor such as ruthenium chloride orruthenium nitrosyl nitrate in a solvent. The solution is contacted to asubstrate heated to a temperature sufficient to, for all intents andpurposes, instantaneously convert the deposited precursor to a highlyporous, high surface area pseudocapacitive film of ruthenium oxideprovided on the substrate.

The prior art describes various methods of contacting the substrate withthe semiconductive or pseudocapacitive solution, or precursor thereof.Commonly used techniques include dipping and pressurized air atomizationspraying of the pseudocapacitive material onto the substrate.Capacitance values for electrodes made by dipping, pressurized airatomization spraying and sputtering are lower in specific capacitance.Sol-gel deposition is another conventional method of coating thesubstrate. It is exceptionally difficult to accurately control thecoating morphology due to the controllability and repeatability of thevarious prior art techniques, which directly impacts capacitance.

Therefore, while electrochemical capacitors provide much higher energystorage densities than conventional capacitors, there is a need tofurther increase the energy storage capacity of such devices. One way ofaccomplishing this is to provide electrodes which can be manufacturedwith repeatably controllable morphology according to the presentinvention, in turn benefiting repeatably increased effective surfaceareas.

SUMMARY OF THE INVENTION

The present invention describes the deposition of an ultrasonicallygenerated, aerosol spray of a pseudocapacitive metal compound or aprecursor of the compound onto a heated conductive substrate The heatedsubstrate serves to instantaneously solidify the compound and in thecase of the solution containing a precursor, convert the precursor tothe pseudocapacitive metal compound provided on the substrate in a solidform. When a liquid is ultrasonically atomized, the resultant dropletsare much smaller in size than those produced by a pressurized airatomizer and the like, i.e., on the order of microns and submicrons incomparison to predominately tens to hundreds of microns, which resultsin a greater surface area coating. Therefore, the capacitance ofpseudocapacitors can be further improved by using an electrode coatedwith an ultrasonically deposited porous film to increase the surfacearea of the electrodes. Additionally, depositing the aerosol onto aheated substrate results in fewer process steps, minimization ofcontamination of the coating by reducing process locations, increasedsurface area for the coating by reducing moisture absorption, and thelike. The benefits result in a coated substrate that is useful as anelectrode in a capacitor and the like having increased energy storagecapacity.

These and other aspects of the present invention will become moreapparent to those skilled in the art by reference to the followingdescription and the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an ultrasonic aerosol depositionapparatus 10 according to the present invention.

FIG. 2 is a schematic of a unipolar electrode configuration for use inan electrochemical capacitor.

FIG. 3 is a schematic of a bipolar electrode configuration for use in anelectrochemical capacitor.

FIG. 4 is a schematic of a hybrid capacitor according to the presentinvention.

FIG. 5 is a schematic of a spirally wound configuration for use in aelectrochemical capacitor.

FIGS. 6 and 7 are photographs taken through an electron microscope at500x and 5,000x, respectively, showing the surface condition of aruthenium oxide coating produced by pressurized air atomization sprayingaccording to the prior art.

FIGS. 8 and 9 are photographs taken through an electron microscope at500x and 5,000x, respectively, showing the surface condition of aruthenium oxide coating produced from an ultrasonically generatedaerosol/mist according to the present invention.

FIG. 10 is a graph of the direct current capacitance of capacitors builtaccording to the present invention in comparison to capacitors accordingto the prior art using the cyclic voltammetry technique.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 illustrates a preferred ultrasonicaerosol deposition apparatus 10 according to the process of the presentinvention. While not shown in the figure, the first step in the processincludes providing a solution of reagents that are intended to be formedinto an ultrasonically generated aerosol according to the presentinvention. The reagent solution is fed into or otherwise provided in areagent chamber 12 via a feed line 14. The reagent solution preferablycontains ions in substantially the ratio needed to form the desiredcoating from the ultrasonically generated aerosol. These ions arepreferably available in solution in water soluble form such as in watersoluble salts. However, salts including nitrates, sulfates andphosphates of the cations which are soluble in other solvents such asorganic and inorganic solvents may be used. Water soluble salts includenitrates and chlorides. Other anions which form soluble salts with thecations also may be used.

The reagent solution in the chamber 12 is moved through a conduit 16 toan ultrasonic nozzle 18. The reagent solution is caused to spray fromthe nozzle 18 in the form of an aerosol 20, such as a mist, by anyconventional means which causes sufficient mechanical disturbance of thereagent solution. In this description, the term aerosol 20 refers to asuspension of ultramicroscopic solid or liquid particles in air or gashaving diameters of from about 0.1 microns to about 100 microns andpreferably less than about 20 microns. Examples include smoke, fog andmist. In this description, the term mist refers to gas-suspended liquidparticles having diameters less than about 20 microns.

In the preferred embodiment of the present invention, the aerosol/mist20 is formed by means of mechanical vibration including ultrasonic meanssuch as an ultrasonic generator (not shown) provided inside reagentchamber 12. The ultrasonic means contacts an exterior surface of theconduit 16 and the ultrasonic nozzle 18 assembly. Electrical power isprovided to the ultrasonic generator through connector 22. As is knownto those skilled in the art, ultrasonic sound waves are those havingfrequencies above 20,000 hertz. Preferably, the ultrasonic power used togenerate the aerosol/mist 20 is in excess of one-half of a watt and,more preferably, in excess of one watt. By way of illustration, anultrasonic generator useful with the present invention is manufacturedby Sonotek of Milton, N.Y. under model no. 8700-120MS.

It should be understood that the oscillators (not shown) of theultrasonic generator may contact an exterior surface of the reagentchamber 12 such as a diaphragm (not shown) so that the producedultrasonic waves are transmitted via the diaphragm to effect misting ofthe reagent solution. In another embodiment of the present invention,the oscillators used to generate the aerosol/mist 20 are in directcontact with the reagent solution. The reagent chamber 12 may be anyreaction container used by those skilled in the art and shouldpreferably be constructed from such weak acid-resistant materials astitanium, stainless steel, glass, ceramic and plastic, and the like.

As the aerosol/mist 20 sprays from the ultrasonic nozzle 18, the sprayis contained by a shroud gas represented by arrows 24. The shroud gas 24does not contact the reagent solution prior to atomization, but insteadsprays from a plurality of shroud gas nozzles (not shown) supported byan air shroud chamber 26 serving as a manifold for the nozzles disposedin an annular array around the ultrasonic nozzle 18. The shroud gas 24is introduced into the air shroud chamber 26 via feed line 28 anddischarges from the shroud gas nozzles at a flow rate sufficient toscreen and direct the aerosol/mist 20 toward a heated substrate 30supported on a holder or a support block 32. For example, with theaerosol/mist 20 spraying from the ultrasonic nozzle 18 at a flow rate offrom about 0.1 cc to 10 cc per minute, the flow rate of the shroud gas24 is from about 500 cc to about 25 liters per minute.

Substantially any gas which facilitates screening, directing and shapingthe aerosol 20 may be used as the shroud gas 24. For example, the shroudgas may comprise oxygen, air, argon, nitrogen, and the like. It ispreferred that the shroud gas 24 be a compressed gas under a pressure inexcess of 760 millimeters of mercury. Thus, the compressed shroud gas 24facilitates the spraying of the aerosol/mist 20 from the ultrasonicnozzle 18 onto the substrate 30.

Substrate 30 preferably consists of a conductive metal such as titanium,molybdenum, tantalum, niobium, cobalt, nickel, stainless steel,tungsten, platinum, palladium, gold, silver, copper, chromium, vanadium,aluminum, zirconium, hafnium, zinc and iron, and the like, and mixturesand alloys thereof.

Regardless of the material of substrate 30, ultrasonically depositedspray coatings rely mostly upon mechanical bonding to the substratesurface. It is, therefore, critical that the substrate surface to becoated is properly prepared to ensure coating quality. For one,substrate surface cleanliness is very important in all coating systems,especially in ultrasonically deposited spray coatings. In that respect,it is required that the substrate surface remain uncontaminated bylubricants from handling equipment or body oils from hands and the like.Substrate cleaning includes chemical means such as conventionaldegreasing treatments using aqueous and nonaqueous solutions as wellknown to those skilled in the art. Plasma cleaning is also contemplatedby the scope of the present invention.

After substrate surface cleaning, surface roughness is the next mostcritical factor for properly applying an ultrasonically deposited spraycoating. The substrate 30 may be roughened by chemical means, forexample, by contacting the substrate with hydrofluoric acid and/orhydrochloric acid containing ammonium bromide and methanol and the like,by plasma etching, and by mechanical means such as scraping, machining,wire brushing, rough threading, grit blasting, a combination of roughthreading then grit blasting and abrading such as by contacting thesubstrate with Scotch-Brite™ abrasive sheets manufactured by 3M.

It is further contemplated by the scope of the present invention that,if desired, the electrical conductivity of substrate is improved priorto coating. Metal and metal alloys have a native oxide present on theirsurface. This is a resistive layer and hence, if the material is to beused as a substrate for a capacitor electrode, the oxide is preferablyremoved or made electrically conductive prior to deposition of apseudocapacitive coating thereon. In order to improve the electricalconductivity of the substrate, various techniques can be employed. Oneis shown and described in U.S. Pat. No. 5,098,485 to Evans, thedisclosure of which is hereby incorporated by reference. A preferredmethod for improving the conductivity of the substrate includesdepositing a minor amount of a metal or metals from Groups IA, IVA andVIIIA of the Periodic Table of Elements onto the substrate. Aluminum,manganese, nickel and copper are also suitable for this purpose. Thedeposited metal is then "intermixed" with the substrate material by, forexample, a high energy ion beam or a laser beam directed towards thedeposited surface. These substrate treating processes are performed atrelatively low temperatures to prevent substrate degradation anddeformation. Additionally, these treating processes can be used topassivate the substrate from further chemical reaction while stillproviding adequate electrical conductivity. For additional disclosureregarding improving the electrical conductivity of the substrate 30prior to deposition, reference is made to simultaneously filed patentapplication entitled "Method of Improving Electrical Conductivity ofMetals, Metal Alloys and Metal Oxides" U.S. patent application Ser. No.08/847,946, which is assigned to the present invention and incorporatedherein by reference.

The reagent solution preferably contains ions in substantially thestoichiometric ratio needed to form the desired coating. In oneembodiment, the ions are present in the reagent solution in awater-soluble form as water-soluble salts. Suitable water-soluble saltsinclude nitrates and chlorides of the cations Alternatively, salts suchas sulfates and phosphates soluble in organic and inorganic solventsother than water may be used. Some of these other solvents includeisopropyl alcohol and nitric acid and the like, and mixtures thereof.

The aerosol/mist contacted substrate 30 consists essentially of a porousfilm coating (not shown) including the oxide of a first metal, or aprecursor thereof, the nitride of the first metal, or a precursorthereof, the carbon nitride of the first metal, or a precursor thereof,and/or the carbide of the first metal, or a precursor thereof, theoxide, nitride, carbon nitride and carbide of the first metal havingpseudocapacitive properties. The first metal is preferably selected fromthe group consisting of ruthenium, cobalt, manganese, molybdenum,tungsten, tantalum, iron, niobium, iridium, titanium, zirconium,hafnium, rhodium, vanadium, osmium, palladium, platinum, and nickel. Forexample, in the case where it is intended that the resultingpseudocapacitive film is an oxide of one of the above listed firstmetals, the deposited mixture can include a nitrate or a chloride of themetal.

The porous coating may also include a second or more metals. The secondmetal is in the form of an oxide, a nitride, a carbon nitride or acarbide, or precursors thereof and is not essential to the intended useof the coated foil as a capacitor electrode and the like. The secondmetal is different than the first metal and is selected from one or moreof the group consisting of tantalum, titanium, nickel, iridium,platinum, palladium, gold, silver, cobalt, molybdenum, ruthenium,manganese, tungsten, iron, zirconium, hafnium, rhodium, vanadium,osmium, and niobium. In a preferred embodiment of the invention, theporous coating product includes oxides of ruthenium and tantalum.

In general, as long as the metals intended to comprise the coating arepresent in solution in the desired stoichiometry, it does not matterwhether they are present in the form of a salt, an oxide, or in anotherform. However, preferably the solution contains either the salts of thecoating metals, or their oxides

The reagent solution is preferably at a concentration of from about 0.01to about 1,000 grams of the reagent compounds per liter of the reagentsolution. In one embodiment of the present invention, it is preferredthat the reagent solution has a concentration of from about 1 to about300 grams per liter and, more preferably, from about 5 to about 40 gramsper liter.

The support block 32 for substrate 30 is heated via a power cable 34. Inthe case where the reagent solution contains a pseudocapacitive metalcompound and during the ultrasonic spray deposition of the aerosol/mist20 onto the substrate 30, support block 32 maintains the substrate 30 ata temperature sufficient to instantaneously evaporate or otherwise driveoff the solvent from the deposited reagent mixture. When the depositedfilm coating is comprised of a precursor of the pseudocapacitive metalcompound, the support block 32 maintains the substrate 30 at atemperature sufficient to instantaneously convert the precursor to aporous, high surface area metal oxide, metal nitride, metal carbonnitride or metal carbide coating on the substrate 30, as the case maybe.

Thus, as the substrate 30 is being coated with the pseudocapacitivemetal solution, or precursor thereof, the substrate is at a temperaturesufficient to drive off or otherwise evaporate the solvent material toprovide a solid, anhydrous form of the pseudocapacitive metal compoundon the substrate. According to the present invention, the solvent isinstantaneously evaporated from the aerosol/mist 20 with contact to thesubstrate resulting in the deposition of a relatively thin film coatingof an oxide of the first metal. In the case of the solution containing aprecursor of the pseudocapacitive metal compound, the heated substratealso instantaneously converts the precursor to the final product inaccordance to the present invention.

According to the present invention, when the resulting film is intendedto be an oxide, the deposited nitrate or chloride mixture isinstantaneously heated by contact with the substrate provided at atemperature sufficient to convert the deposited precursor to a highlyporous, high surface area pseudocapacitive film. More particularly, asthe oxide precursor aerosol/mist 20 is spraying onto the heatedsubstrate 30, the substrate is at a temperature of about 100° C. toabout 500° C., preferably about 350° C. to instantaneously convert theprecursor to an oxide coating. After deposition and conversion to thepseudocapacitive compound, the substrate may be ramped down or cooled toambient temperature, maintained at the heated deposition temperature, orvaried according to a specific profile. In general, it is preferred toconduct this heating while contacting the substrate with air or anoxygen-containing gas.

Alternatively and as described is simultaneously filed patentapplication entitled "Ultrasonically Coated Substrate For Use In ACapacitor And Method of Manufacture" U.S. patent application Ser. No.08/858,150, the ultrasonically generated aerosol is sprayed onto thesubstrate maintained at a temperature sufficient to evaporate orotherwise drive off the solvent from the deposited reagent mixture. Whenthe deposited film coating is comprised of a precursor of thepseudocapacitive melted compound, the coated substrate is then subjectedto a separate heating step to convert the precursor to the finalproduct. The simultaneously filed patent application is assigned to theassignee of the present invention and incorporated herein by reference.

It is preferred that the resulting porous coating, whether it be of anoxide, a nitride, a carbon nitride or a carbide, have a thickness offrom about a few hundred Angstroms to about 0.1 millimeters or more. Theporous coating has an internal surface area of about 10 m² /gram toabout 1,500 m² /gram. In general, the thickness of substrate 30 istypically in the range of about 0.001 millimeters to about 2 millimeterand preferably about 0.1 millimeters.

During aerosol/mist 20 deposition, temperature sensing means (not shown)are used to sense the temperature of the substrate 30 and to adjust thepower supplied to the support block 32 to regulate the substratetemperature as previously described.

One advantage of the present process is that the substrate 30 may be ofsubstantially any size or shape, and it may be stationary or movable.Because of the speed of the coating process, the substrate 30 may bemoved across the spray emitting from nozzle 18 to have any or all of itssurface coated with the film. The substrate 30 is preferably moved in aplane which is substantially normal to the direction of flow of theaerosol region 20. In another embodiment, the substrate 30 is movedstepwise along a predetermined path to coat the substrate only atcertain predetermined areas. In another embodiment of the presentprocess, rotary substrate motion is utilized to expose the surface of acomplex-shaped article to the aerosol coating. This rotary substratemotion may be effected by conventional means.

The process of the present invention provides for coating the substrate30 at a deposition rate of from about 0.01 to about 10 microns perminute and, preferably, from about 0.1 to about 1.0 microns per minute.The thickness of the film coated upon the substrate 30 may be determinedby means well known to those skilled in the art.

The present aerosol spray deposition process provides a substantialamount of flexibility in varying the porosity and morphology of thedeposited film. By varying such parameters as the concentration of thereagent solution (a higher concentration of the metal constituentsproduces a larger particle size as well as a higher deposition rate),the temperature of the substrate (the higher the substrate temperature,the larger the size of the grains deposited), energy supplied by theultrasonic generator (the greater the energy, the faster the depositionrate) and ultrasonic frequency (the higher the frequency, the smallerthe particle size resulting in a higher surface area aerosol depositedfilm), the porosity and morphology of the deposited film coated onto thesubstrate 30 is controlled. Also, the temperature of the substrateaffects the crystal structure and coating adhesion strength.

It is preferred that the generation of the aerosol/mist 20 and itsdeposition onto the substrate 30 is conducted under substantiallyatmospheric pressure conditions. As used in this specification, the term"substantially atmospheric" refers to a pressure of at least about 600millimeters of mercury and, preferably, from about 600 to about 1,000millimeters of mercury. It is preferred that the aerosol generationoccurs at about atmospheric pressure. As is well known to those skilledin the art, atmospheric pressure at sea level is 760 millimeters ofmercury.

An ultrasonically coated substrate according to the present invention isuseful as an electrode in various types of electrochemical capacitorsincluding unipolar and bipolar designs, and capacitors having a spirallywound configuration. For example, in FIG. 2 there is shown a schematicrepresentation of a typical unipolar electrochemical capacitor 40 havingspaced apart electrodes 42 and 44 One of the electrodes, for example,electrode 42, serves as the cathode electrode and comprises anultrasonically generated aerosol coating 46A of pseudocapacitivematerial provided on a conductive plate 48A according to the presentinvention. For example, a porous ruthenium oxide film is provided onplate 48A which is of a conductive material such as tantalum. Therelative thicknesses of the plate 48A and the pseudocapacitive coating46A thereon are distorted for illustrative purposes. As previouslydescribed, the plate is about 0.01 millimeters to about 1 millimeter inthickness and the pseudocapacitive coating 46A is in the range of abouta few hundred Angstroms to about 0.1 millimeters thick. The otherelectrode 44 serves as the anode and is of a similar pseudocapacitivematerial 46B contacted to a conductive substrate 48B, as in electrode42.

The cathode electrode 42 and the anode electrode 44 are separated fromeach other by an ion permeable membrane 50 serving as a separator. Theelectrodes 42 and 44 are maintained in the spaced apart relationshipshown by opposed insulating members 52 and 54 such as of an elastomericmaterial contacting end portions of the plates 48A, 48B. The end plateportions typically are not coated with a pseudocapacitive material. Anelectrolyte (not shown), which may be any of the conventionalelectrolytes used in electrolytic capacitors, such as a solution ofsulfuric acid, potassium hydroxide, or an ammonium salt is providedbetween and in contact with the cathode and anode electrodes 42 and 44.Leads (not shown) are easily attached to the electrodes 42 and 44before, during, or after assembly of the capacitor and the thuslyconstructed unipolar capacitor configuration is housed in a suitablecasing, or the conductive plates along with the insulating members canserve as the capacitor housing.

FIG. 3 is a schematic representation of a typical bipolarelectrochemical capacitor 60 comprising a plurality of capacitor units62 arranged and interconnected serially. Each unit 62 includes bipolarconductive substrate 64. Porous pseudocapacitive coatings 66 and 68 areprovided on the opposite sides of substrate 64 according to the presentultrasonic coating process. For example, a porous coating of rutheniumoxide film is deposited from an ultrasonically generated aerosol ontoboth sides of substrate 64. Again, the thickness of the porous coatings66 and 68 is distorted for illustrative purposes. The units 62 are thenassembled into the bipolar capacitor configuration on opposite sides ofan intermediate separator 70. Elastomeric insulating members 72 and 74are provided to maintain the units 62 in their spaced apartrelationship. Materials other than elastomeric materials may be apparentto those skilled in the art for use as insulators 72, 74. As shown inthe dashed lines, a plurality of individual electrochemical capacitorunits 62 are interconnected in series to provide the bipolarconfiguration. The serial arrangement of units 62 is completed at theterminal ends thereof by end plates (not shown), as is well known tothose skilled in the art. As is the situation with the unipolarcapacitor configuration previously described, an electrolyte 74 isprovided between and in contact with the coatings 66, 68 of thecapacitor 60.

FIG. 4 shows a schematic representation of an electrolytic capacitor 80having spaced apart cathode electrode 82, 84, each comprising arespective ultrasonically generated aerosol coating 82A, 84A ofpseudocapacitive material provided on a conductive plate 82B, 84Baccording to the present invention. The counter electrode or anode 86 isintermediate the cathodes 82, 84 with separators 88, 90 preventingcontact between the electrodes. The anode 86 is of a conventionalsintered, metal, preferably in a porous form. Suitable anode metals areselected from the group consisting of titanium, aluminum, niobium,zirconium, hafnium, tungsten, molybdenum, vanadium, silicon, germaniumand tantalum contacted to a terminal pin 92. The hybrid capacitor 80 iscompleted by insulating members 94, 96 contacting end portions of thecathode plates. While not shown, an electrolyte is provided to activatethe electrodes 82, 84 and 86.

FIG. 5 is a schematic drawing of another embodiment of a jelly rollconfigured capacitor 100, which can be manufactured by the ultrasoniccoating process according to the present invention. Capacitor 100 has aplurality of capacitor units 102, each comprising a conductive substrateprovided with ultrasonically generated pseudocapacitive coatings 104,106 on the opposed sides thereof. The coatings can be, for example, ofruthenium oxide, separated from immediately adjacent cells by anintermediate separator 108. This structure is then wound in a jelly rollfashion and housed in a suitable casing. Leads are contacted to theanode and cathode electrodes and the capacitor is activated by anelectrolyte in the customary manner.

The following example describes the manner and process of coating asubstrate according to the present invention, and they set forth thebest mode contemplated by the inventors of carrying out the invention,but they are not to be construed as limiting.

EXAMPLE I

A precursor solution was prepared by dissolving 2.72 grams of rutheniumnitrosyl nitrate in a solvent that consisted of 100 cc of deionizedwater. If needed, a minor amount, i.e. about 5 cc of nitric acid is usedto completely solubilize the precursor. The solution was stirred untilthe ruthenium nitrosyl nitrate was completely dissolved. ABecton-Dickinson, 30 cc. syringe was filled with the precursor solutionand installed in the syringe pump. The pump was set to an injection flowrate of 2 cc/minute. The ruthenium precursor solution was then ready tobe sprayed using the ultrasonic aerosol generator (Sonotek).

The substrate was cleaned with appropriate cleaning solutions andmounted on the temperature controlled substrate holder. The substratewas a tantalum foil, 0.002" thick. The foil was heated to a temperatureof 350° C. The ultrasonic nozzle was positioned above the substrate at aheight of 7 cm. The power to the nozzle was set to 1.0W. The shroud gas,dry and filtered air, was turned on and set to a flow rate of 15 scfh at10 psi. This shroud gas behaves as the carrier gas and also acts as anaerosol mist shaping gas. After the foil temperature stabilized thesyringe pump was turned on. As the liquid precursor was pumped throughthe nozzle it was atomized into tiny droplets. The droplets weredeposited on the heated substrate where the solvent evaporated and aruthenium nitrosyl nitrate film was created on the surface of the foil.The foil temperature of 350° C. was sufficient to convert the rutheniumnitrosyl nitrate to ruthenium oxide as the aerosol was being deposited.On completion of the spraying, the film was allowed to remain on theheater block for half an hour in order to ensure that all the nitratehad been converted to the oxide.

CONCLUSION

When a liquid is ultrasonically atomized, the droplet size in theaerosol/mist is smaller than that produced by the various prior arttechniques previously discussed. This results in greater control overthe manufacturing process in terms of controlling the coating morphologyfrom one production run to the next. Also, there is less oversprayingwith the present process in comparison to pressurized air atomizationspraying and the like. Furthermore, the use of an ultrasonicallygenerated aerosol deposited on a conductive substrate to form anelectrode for a capacitor according to the present invention provides ahigher surface area coating than that obtainable by the prior art, andthus a higher capacitance electrode.

FIGS. 6 and 7 are photographs taken through an electron microscope at500x and 5,000x, respectively, showing the surface condition of aruthenium oxide coating produced by dipping according to the prior art.FIGS. 8 and 9 are photographs taken through an electron microscope at500x and 5,000x, respectively, showing the surface condition of aruthenium oxide coating produced from an ultrasonically generatedaerosol/mist according to the present invention.

As is apparent, the film morphology of the present coatings is differentthan that of the prior art coating. The prior art coatings have a"cracked mud" appearance while the present coatings have the same"cracked mud" appearance plus additional structures on the "cracked mud"area. The cracks of the present coatings are also higher in density andthus they have an increased surface area.

FIG. 10 is a graph of the current versus voltage of various capacitorsbuilt according to the present invention and built according to theprior art. The present invention capacitors contained electrodes madeaccording to Example I. The prior art capacitors contained electrodesmade by high pressure air atomization or nebulization of a rutheniumchloride solution that was subsequently heated to form a ruthenium oxidecoating. In particular, curves 110 and 112 where constructed from thecyclic voltammetry scans of the respective present invention capacitorswhile curves 114 and 116 were constructed from the cyclic voltammetryscans of the two prior art capacitors. The scan rate was 10 mV/sec.

It has been determined that the capacitance obtained from a capacitorhaving an electrode made according to the present invention is in therange of about 50 to 900 Farad/gram (F/g.) of coating material asmeasured by the cyclic voltammetry technique. The prior art capacitorsused to construct curves 114 and 116 in FIG. 10 had capacitances ofabout 75 F/g. measured by the same technique.

It is appreciated that: various modifications to the inventive conceptsdescribed herein may be apparent to those skilled in the art withoutdeparting from the spirit and the scope of the present invention definedby the hereinafter appended claims.

What is claimed is:
 1. A capacitor, which comprises:a) a casing; b) acathode comprising a porous coating of at least a first pseudocapacitivemetal compound provided on a substrate, wherein the coating ischaracterized as comprising particles formed from an ultrasonicallygenerated aerosol of either the first pseudocapacitive metal compound,or a precursor thereof contacted to the substrate heated to atemperature to instantaneously solidify the first pseudocapacitive metalcompound or convert the precursor to a solidified pseudocapacitive metalcompound; c) an anode spaced from the porous cathode coating; and d) anelectrolyte in contact with the porous cathode coating and the anode,the casing containing the anode, the cathode and the electrolyte.
 2. Thecapacitor of claim 1 wherein the first pseudocapacitive metal compoundis selected from the group consisting of an oxide, a nitride, a carbonnitride and a carbide, and mixtures thereof.
 3. The capacitor of claim 1wherein a majority of the particles have diameters of less than about 10microns.
 4. The capacitor of claim 1 wherein an internal surface area ofthe coating is about 10 m² /gram to about 150 m² /gram.
 5. The capacitorof claim 1 wherein the coating has a thickness of about a hundredAngstroms to about 0.1 millimeters.
 6. The capacitor of claim 1 whereinthe casing comprises a metal and the porous coating is provided directlyon an inside surface of the casing.
 7. The capacitor of claim 1 whereina first metal of the pseudocapacitive compound is selected from thegroup consisting of ruthenium, molybdenum, tungsten, tantalum, cobalt,manganese, nickel, iridium, iron, titanium, zirconium, hafnium, rhodium,vanadium, osmium, palladium, platinum and niobium, and mixtures thereof.8. The capacitor of claim 1 wherein the porous coating includes amixture of at least one oxide chosen from the group consisting of oxidesof ruthenium, molybdenum, tungsten, tantalum, cobalt, manganese, iron,iridium, nickel, titanium, zirconium, hafnium, rhodium, vanadium,osmium, palladium, platinum and niobium, and at least one oxide chosenfrom the group consisting of oxides of tantalum, titanium, nickel,iridium, platinum, palladium, gold, silver, cobalt, molybdenum, niobium,ruthenium, manganese, tungsten, iron, zirconium, hafnium, rhodium,vanadium and osmium.
 9. The capacitor of claim 1 wherein the porouscoating includes a mixture of oxides of ruthenium and tantalum.
 10. Thecapacitor of claim 1 wherein the porous coating is formed by depositingthe ultrasonically generated aerosol of the precursor of the firstpseudocapacitive metal compound to the heated substrate in air or anoxygen containing atmosphere.
 11. The capacitor of claim 1 wherein theanode is selected from the group consisting of tantalum, aluminum,niobium, zirconium and titanium, and mixtures thereof.
 12. The capacitorof claim 1 wherein the anode is of a pseudocapacitive metal compound.13. The capacitor of claim 1 is an electrochemical capacitor.
 14. Thecapacitor of claim 1 having an unipolar configuration.
 15. The capacitorof claim 1 having a bipolar configuration.
 16. The capacitor of claim 1having a spirally wound configuration.
 17. The capacitor of claim 1 isan electrolytic capacitor.
 18. The capacitor of claim 17 wherein theanode is selected from the group consisting of tantalum, titanium,aluminum, niobium, zirconium, hafnium, tungsten, molybdenum, vanadium,silicon and germanium, and mixtures thereof.